Forming a conductive pattern on a substrate

ABSTRACT

A method of forming a conductive pattern on a substrate. The method comprising providing a substrate carrying a conductive layer; forming a first portion of the conductive pattern by exposing the conductive layer to a laser and controlling the laser to remove conductive material around the edge(s) of desired conductive region(s) of the first portion; and laying down an etch resistant material on the conductive layer, the etch resistant material defining a second portion of the conductive pattern, removing conductive material from those areas of the second portion not covered by the etch resistant material, and then removing the etch resistant material.

The invention relates to a method of forming a conductive pattern on asubstrate, for example so as to form an electronic circuit.

In one approach to forming very high resolution electrical circuits, forexample that used by LPKF Laser and Electronics AG of Germany, a laseris used to cut a narrow line through a layer of a conductive material onan insulating substrate. Complete loops of such cuts are used to isolateareas of conductor which then become the conductive traces of thecircuit. This process is useful for forming prototype circuit boards butis slow and the resulting circuit has large areas of unwanted conductorremaining. These are normally left in place since it would take a verylong time to use the laser to remove them.

An alternative approach which can be used for low resolution circuits isto define the required circuit as a rasterised image which is thenprinted, typically using an ink jet printer, over the whole area of theconductive layer using an ink capable of resisting a chemical etch. Therequired circuit pattern is then produced by etching the conductivelayer where it is not covered with the ink, and then removing the inkusing another chemical such as a solvent. This process is suitable forproducing small volume or prototype products as it is reasonably fastand economical with materials but is not capable of high resolution.This is because of the rasterisation process and the unpredictablebehaviour of the printing heads and of the ink on the surface.

In accordance with the present invention, a method of forming aconductive pattern on a substrate comprises:

-   -   a) providing a substrate carrying a conductive layer;    -   b) forming a first portion of the conductive pattern by exposing        the conductive layer to a laser and controlling the laser to        remove conductive material around the edge(s) of desired        conductive region(s) of the first portion; and,    -   c) laying down an etch resistant material on the conductive        layer, the etch resistant material defining a second portion of        the conductive pattern, removing conductive material from those        areas of the second portion not covered by the etch resistant        material, and then removing the etch resistant material.

We have realized that typical circuits have some areas requiring highresolution, for example tracks around high density Ball Grid Array (BGA)chips, and other, normally larger areas, where a lower resolution isadequate. It is then possible to optimize the use of the two techniquesso that the laser process is only used in those areas requiring highresolution and the etch process is used in the remaining, low resolutionareas.

In some cases, the etch resistant material could be confined to thesecond portion but this has the disadvantage that it is necessary toprevent the removal chemical from reaching the first portion.Preferably, therefore, the etch resistant material at least overlaps theconductive regions of the first portion and preferably is laid down as asolid block over the whole of the first portion.

Typically, the etch resistant material will be printed, for exampleusing an ink jet printer or other conventional printer.

An example of a method according to the invention will now be describedwith reference to the accompanying drawings, in which:—

FIGS. 1A and 1B are a plan and schematic cross-section of a substratecarrying a conductive layer; and,

FIGS. 2 to 5 illustrate plan views of the conductive layer at successivestages during the process.

FIGS. 1A and 1B illustrate an insulating substrate 1 on which isprovided a continuous, conductive, e.g. copper, surface layer 2.

In the first stage of the process, a first portion 3 of the copper layer2 is exposed to a laser beam (not shown). A suitable beam could begenerated from a neodymium YAG or frequency-doubled YAG laser. The laserbeam is controlled to define the edges of desired conductive regions4A-4G by removing or ablating thin lines of the conducting material asshown at 5A-5G. Portions of the substrate 1 can be seen through theregions 5A-5G. A resolution better than 100 μm is achieved, even to 25μm.

In the next step (FIG. 3), an etch resist ink 20, such as Jetrack EtchResist 001 from Avecia Electronic Materials, is applied using an ink jetprinter (not shown) onto the surface of the copper layer 2. In theportion 6, this etch resist ink defines the desired circuit pattern at arelatively low resolution, e.g. 100 μm or more. In the portion 3, theetch resist ink is laid down in regions 10A-10C on previously definedconductive regions 4A-4C so as slightly to overlap the correspondingetch lines 5A-5C. In addition, a solid block of the etch resist ink 10Dis laid down over all the regions 4D-4G. Finally, the etch resist ink isalso laid down so as to define connection tabs 11A-11C.

In the next stage, a chemical etchant, such as ferric chloride, isapplied to the entire surface of the conductive material 2 and thisremoves all exposed regions of copper to leave the etch resist ink aloneas can be seen in FIG. 4. (The edge of the substrate 1 is omitted inFIG. 4.)

Finally, the etch resist ink is removed using a conventional solvent,such as acetone or sodium hydroxide solution, so as to leave thepatterned copper only, as can be seen in FIG. 5. It will be seen in FIG.5 that some regions of copper 12A-12C are left and these are redundant.

Where the laser-defined pattern meets the injet-defined pattern therewill need to be some small overlap to take up any registration errorbetween the two processes. For example, the laser cuts could extend forsome distance into the area to be patterned by inkjet. Alternatively, itis possible to control the placing of etch resist ink using thedifferent wetting properties of the ink on the conducting layer and onthe insulating layer revealed when the conducting layer is removed; thisopens the possibility of using laser cuts to locate inkjet printedmaterial more accurately in the overlap region.

The laser and ink jet printer could be mounted on a single machinecapable of X-Y vector and raster scanning, or two separate machinescould be used.

1. A method of forming a conductive pattern on a substrate, the methodcomprising: a) providing a substrate carrying a conductive layer; b)forming a first portion of the conductive pattern by exposing theconductive layer to a laser and controlling the laser to removeconductive material around the edge(s) of desired conductive region(s)of the first portion; and, c) laying down an etch resistant material onthe conductive layer, the etch resistant material defining a secondportion of the conductive pattern, removing conductive material fromthose areas of the second portion not covered by the etch resistantmaterial, and then removing the etch resistant material.
 2. A methodaccording to claim 1, wherein the etch resistant material is laid downalso over the first portion of the conductive pattern in step c).
 3. Amethod according to claim 2, wherein the etch resistant materialoverlaps the conductive region(s) of the first portion.
 4. A methodaccording to claim 2, wherein the etch resistant material is laid downas a solid block over the first portion.
 5. A method according to claim1, wherein the first portion overlaps the second portion.
 6. A methodaccording to claim 1, wherein the first portion of a conductive patternis formed at a higher resolution than the second portion of theconductive pattern.
 7. A method according to claim 6, wherein the firstportion of the conductive pattern has features defined to a resolutionof 100 μm or less, while the second portion has features defined to aresolution of more than 100 μm.
 8. A method according to claim 1,wherein the etch resistant layer is printed in step c).
 9. A methodaccording to claim 8, wherein the etch resistant material is ink jetprinted.